Well, son of a gun. I did search on the manual for 28U, and sure enough, there is no instruction to solder it. Yes the terminal and its wires should be soldered, and a missing solder joint there will increase the possibility of noise getting int the signal path. Note that, because of the large number of wires and the fact that the terminal is bolted to the chassis, it may take more heat than usual to get a good connection.
There is an instruction to solder 28L, and as far as I can tell that would be a good time to get 28U as well - in fact it may be quite difficult to NOT get 28U at that point!